Process of cleaning metal



Patented Aug. 16, 1932 UNITED STATES PATENT OFFICE HARRY C. MOUGEY, OFDETROIT, MICHIGAN, ASSIGNOR TO GENERAL MOTORS RE- SEARCH CORPORATION,gDETROIT, MICHIGAN, A CORPORATION OF DELAWARE PROCESS OF CLEANING METALNo Drawing.

The invention relates to methods of treating metal surfaces especiallyof iron or steel to prepare them for the application of protective ordecorative coatings such as paint, lacquer or enamel. Such coatingsapplied in liquid form and dried by evaporation of solvents or otherwiseare termed herein, for convenience, paint coatings. The primary objectis to provide a treatment preliminary to the application of the coatingwhereby the formation of rust or oxide under the coating will beprevented.

It is well known that, when salts of alkali metal which dissociate insolution are brought into contact with a metal surface such as iron orsteel in the presence of water, electrolytic action is set up whichproduces in the cathodic area an alkaline condition and in the anodicarea an acid condition. Both of these conditions are destructive of thecoating if they occur on the surface of the metal beneath the coating.The alkali tends to loosen the coating film from the metal and the acidcauses or promote corrosion and oxidation of the metal. 4

These destructive effects contribute each to the development of theother so that a very small quantity of the alkali metal salts on themetal surface may be sufficient to produce, even within a day or two, anoticeable scaling or blistering of the coating and corrosion or rustingof the metal. In fact, I have found that ordinary industrial watersusually contain suflicient salts of the alkali metals so that, if suchwaters are used for washing or rinsing the metal surface preliminary tocoating with enamel or the like, the drying oif of the water in theusual manner will leave enough alkali metal salts, chiefly sodium salts,to produce the destructive effects above mentioned. A

In accordance with the present invention, I prevent such effects byconverting the alkali metal salts normally present in or, deposited fromordinary industrial waters, as well as other salts present in the waterwhich may form corrosion-promoting deposits, into salts which dissociatein the presence of moisture to form rust-inhibitive compounds,preferably phosphates. For this purpose it is sufficient to add to thefinal rinsing water a very Applicationflled September 24, 1930. SerialNo. 484,250.

small quantity of phosphoric acid which, upon drying the surface,converts the slight residue of corrosion-promoting or rust-stimulatingsalts into rust-inhibitive phosphates. These phosphates are in no waydetrimental to the subsequently applied coating and need not be removed,although it may be desirable to go over the surface lightly with a tackrag i. e., a rag made slightly tacky with varnish, to remove any powderor dust that may be loose on the surface.

The amount of phosphoric acid used in the final rinse water may bevariedsomewhat with the character of the water but the variation in thesalt content of usual industrial waters is relatively so insignificantand the amount of phosphoric acid necessary is so small that variationsin the quality of water may generally be disregarded. The proportion ofacid recommended is not more than about one fourth of one percent of theamount of rinse water. With most city Water supplies in the NorthCentral States, a phosphoric acid addition of two hundredths of onepercent is sufiicient. Ordinary commercial phosphoric acid of about 85%strength is satisfactory for the purpose. If the available suppy ofindustrial water is so impure that the proportion of acid specified isinsufficient to counteract the effect of the undesirable salts, suchwater should not be used for rinse water and condensed steam or otherrelatively pure water should be employed for a final rinse with additionof phosphoric acid as described to counteract any residue from the lastor previous wash waters.

Assuming that the metal surface is free from rust and scale, or has beenfreed therefrom by ordinary pickling processes, after which the surfaceis usually oiled, it is desirable first to wash the surface in some bathadapted to remove oil or grease, such, for example, as a bath containingan alkaline washing compound. Other methods of removal of grease may,however, be employed for example, cleaning with oil solvents such asvolatile hydrocarbons. A subsequent washing, preferably with water, maythen be employed to remove the products of the cleaning operation. Themetal is then dipped or rinsed in the dilute phosphoric acid solutionand, after drying, is ready to receive the pamt coating.

The process described is of especial utility 5 in treating steel or ironparts of automobiles to which paint coatings, particularl enamels, aregenerally applied. Great di culty has been met with, hitherto, inproducing on such parts at low cost enamel coatings of satisfactorydurability. Various treatments have been suggested designed to produceon the iron surface a rust-proof coatiii of iron and/or other phosphatesas by t e process known as Parkerizing or by the use of mixtures ofcleaning agents including relatively' strong solutions of phosphoricacid.

The present invention does not contemplate the formation of any materialamount of iron phosphate on the surface nor the use of phosphoric acidto act in any way as a cleaning agent for the metal but merely theneutralization of the corrosion-stimulating salts present in the rinsingwater. The process is therefore very much less expensive and troublesomethan the phosphoric acid treatments hitherto proposed. Nevertheless, ithas proven highly efiective as a remedy for the difliculties encounteredin producing durable paint coatings on surfaces of ferrous metal. Iclaim:

l. The process of preparing ferrous metal surfaces for paint coatingscomprising cleaning the surface then finally rinsing m ordinaryindustrial water containing salts tending upon dissociation in solutionto promote corrosion of the metal to which has been added phosphoricacid in amount substantially only suflicient, to form with the saidsalts rust-lnhibiting phos hates, said amount 40 being between two antwenty-five hundredth of 1%, and then drying the surface.

2. The process of preparing ferrous metal surfaces for paint coatingscomprising cleaning the surface, then finally rinsing water containingsalts tending upon dissociation in solution to romote corrosion of themetal to which has 11 added from two to twentyfive hundredths of onepercent of phosphoric acid, and then drying the surface.

3. The process of pre aring ferrous metal surfaces for reception 0 paintcoatings comprising cleaning the surface with a ents not leavingphosphoric acid on the sur ace, then finally rinsing with wash watercontaining salts tending upon dissociation in solution to stimulateformation of rust to which has been added phosphoric acid in amountbetween two and twenty-five hundredths of one rcent, the quantity ofsuch salts being s ciently low so that the said addition will convertthem into rust-inhibiting phosphates.

In testimony whereof I aflix my si ature.

' HARRY C. MOU EY.

